In a portable information apparatus, such as a smart phone and tablet PC, the thickness, the size, and the weight thereof have further decreased, so that functions to be mounted thereon have been limited in many cases. For example, Patent Document 1 discloses a configuration which enables extension of functions, such as a battery function, by mounting an extension unit having approximately the same shape as that of a portable information apparatus on the back surface of the portable information apparatus.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2017-45043